This invention relates to signal processing systems and more particularly to apparatus for monitoring arcing of brushes in a dynamoelectric machine.
In dynamoelectric machines such as a turbine-driven generator, brushes are used to conduct current to and from collector slip rings or commutators mounted on a rotor. Because of factors such as brush wear or misalignment, slip ring imperfections, or rotor vibrations, arcing may occur between brushes and the rotating slip ring or commutator surface against which the brushes bear during operation. This brush arcing, even if of low voltage potential, may cause deterioration of slip ring surfaces and may, if undetected and corrective action not taken, lead to excessive arcing and considerable damage to slip rings and brushholder riggings, forcing generator outages and expensive repairs. Monitoring of brush arcing is hence of considerable importance, yet accurate detection of brush arcing, particularly low levels thereof, has proven of considerable difficulty in certain generators due to the presence of high-amplitude noise spikes resulting, for example, from commutation of diodes and silicon-controlled rectifiers in the excitation system of the generator and appearing in the same frequency band as the brush arcing to be monitored.
U.S. Pat. Nos. 3,653,019 to Barton et al and 4,058,804 to Sawada et al, assigned to the assignee of the present invention and whose disclosures are incorporated herein by reference thereto, describe systems for monitoring generator brush arcing which eliminate the high-amplitude noise spikes from a composite signal including brush arcing signals and noise spikes through the use of a pulse-forming network and gating network. The pulse-forming network of these inventions reacts to a noise spike by producing a disabling signal which when applied to the gating network blocks passage of the composite signal therethrough for a predetermined time period at least equal to the duration of the noise spike, and (in the case of the improved system of U.S. Pat. No. 4,058,804) passes the composite signal through the gating network at all other times for subsequent detection of brush arcing signals.
While the inventions set forth in these patents greatly facilitate measurement of brush arcing, the gating concept used therein may cause the systems of U.S. Pat. Nos. 3,653,019 and 4,058,804 to miss or fail to detect some brush arcing signals of potential importance to generator performance and durability. This deficiency occurs because in blocking the composite signal from passage through the gating network for the period of the noise spike and its associated ringing, these systems also block any brush arcing signals occurring during this time interval and hence preclude detection thereof. Since the high-frequency noise spikes may occur at frequent intervals, the "arc detection" time lost due to the blocking pulses can represent a significant percentage of the total time interval monitored, e.g., up to 40 percent, and thus the probability of detecting harmful arcing may be less than desired.
The signal monitor of the present invention overcomes this deficiency by mitigating the effect of the noise spikes without the use of blocking pulses or a gating system and permits accurate detection of a wide range of brush arcing signals in dynamoelectric machines.
Accordingly, it is an object of this invention to provide improved apparatus for monitoring arcing of brushes in a dynamoelectric machine.
Another object of the invention is to provide simple brush monitoring apparatus which will continuously detect brush arcing signals, including low amplitude brush arcing signals, in a composite signal which also contains high-amplitude noise spikes in the same frequency band as the brush arcing signals.
A further object of the invention is to provide a brush monitor which will continuously detect brush arcing signals over a wide range of ambient temperatures.
A still further object of the invention is to provide a brush arc monitor which, in addition to achieving the above objects, will automatically compensate for changes in generator noise level.